Wetting properties of materials have interested researchers for decades, due to their relevance to numerous applications. The wetting properties of a material are dictated by its surface chemistry (Emsley, J., Chemical Society reviews, 9(1):91-124 (1980); Wenzel, R. N., Industrial & Engineering Chemistry, 28(8):988-994 (1936)) and its topographic structure (Bhushan, B. et al., Philosophical transactions—Royal Society. Mathematical, Physical and engineering sciences, 367(1894):1631-1672 (2009); Gao, L. and McCarthy, T. Langmuir, 23(18):9125-9127 (2007); Gao, L. and McCarthy, T., Journal of the American Chemical Society, 128(28):9052-9053 (2006); Krupenkin, T. et al., Langmuir, 20(10):3824-3827 (2004)).
Many investigations have been conducted to understand the surface properties of superhydrophobic materials. A superhydrophobic surface is extremely difficult to wet; it typically has a static contact angle higher than 150° and a contact angle hysteresis less than 10°. Wang, S, and Jiang, L., Advanced materials, 19(21):3423-3424 (2007); Men, X. et al., Applied physics. A, Materials science & processing, 98(2):275-280 (2010); Bhushan, B. et al., Philosophical transactions—Royal Society. Mathematical, Physical and engineering sciences, 367(1894):1631-1672 (2009).
Superhydrophobic materials can be utilized as a protective coating for creating a self-cleaning, nonstick surface (e.g., for solar panels) and for preventing biofouling. Scardino, A. J. et al., Biofouling: The Journal of Bioadhesion and Biofilm Research, 25(8):757-767 (2009). They can be used as electrodes to store charge energy in a non-aqueous supercapacitor. They can also be employed to reduce hydrodynamic skin friction drag in laminar and turbulent flow. Rothstein, J., Annual Review of Fluid Mechanics, 42(1):89-109 (2010). Without intending to be bound by theory, the existence of a thin layer of trapped air at the liquid-solid interface is believed to allow a slip velocity at the wall of superhydrophobic material, reducing shear stress or momentum transfer from the flow to the wall. Ou, J. et al. Physics of Fluids, 16:4635-4643 (2004); Min, T.; Kim, J. Physics of Fluids 16:L55-L58 (2004); Daniello, R. J. et al. Physics of Fluids 21, online publ. no. 085103 (2009). This effect can produce advantages at macro- or micro-scale. For example, superhydrophobic materials could reduce fuel consumption of marine vessels and the efficiency of liquid pipelines. They also could be used in drug delivery devices to protect the device or drug from contact with blood, and they could be used to alter the mechanical response of cells.
In recent years, production of synthetic materials that exhibit superhydrophobic behavior has been reported. Among these materials, vertically aligned, multi-walled carbon nanotube arrays have gained enormous attention, due to their simple fabrication process and inherent two-length scale topographic structure. Efforts have been made to modify the surface chemistry of the carbon nanotube arrays so that their wetting properties can be tuned precisely. The carbon nanotube arrays can be made hydrophilic by functionalizing their surfaces with oxygenated surface functional groups that allow hydrogen bonds with water molecules to form or hydrophobic by removing those oxygenated surface functional groups from their surfaces.
Various oxidation processes can be used to functionalize the surface of carbon nanotube arrays, such as high-temperature annealing in air, UV/ozone treatment, oxygen plasma treatment, and acid treatment. Processes like high-temperature annealing in air and oxygen plasma treatment would be very costly to implement in large scale, not to mention highly probable to over-oxidize the carbon nanotube if an incorrect recipe were used. The acid treatment is generally hazardous, making it inconvenient to work with. On the other hand, the UV/ozone treatment is a simple, safe, and cost-efficient method of producing more hydrophilic carbon nanotubes.
However, no analogous simple, safe, cost-efficient process has yet been identified for producing superhydrophobic carbon nanotubes. Previously reported studies suggest that complicated processes are always involved in producing superhydrophobic carbon nanotube arrays. In order to make these arrays superhydrophobic, they have to be coated with non-wetting chemicals such as poly(tetrafluoroethylene) (PTFE), zinc (II) oxide, and fluoroalkylsilane, (Huang, L. et al., The journal of physical chemistry, B, 109(16):7746-7748 (2005); Lau, K. et al., Nano Lett., 3(12):1701-1705 (2003); Feng, L. et al., Advanced materials, 14(24):1857-1860 (2002)) or be modified by plasma treatments, such as CF4, CH4, and NF3. (Hong, Y. and Uhm, H., Applied physics letters, 88(24):244101 (2006); Cho, S. et al., Journal of materials chemistry, 17(3):232-237 (2007)); Balu, B. et al. Langmuir, 24:4785-4790 (2008). However, no prior art has reported a method for producing a superhydrophobic CNT array surface from pure CNTs grown by a simple self-assembly process.
In view of the foregoing, there is a need for a simple, safe, cost-efficient process for producing superhydrophobic carbon nanotubes. Such a process could help to speed the investigation and the commercial application of superhydrophobic carbon nanotubes. The present invention satisfies these and other needs.